Depositional System and Paleogeographic Evolution of the Middle Jurassic Yan'anian Stage in the Central and Southern Ordos Basin

Based on the new stratigraphic division draft and large quantities of drilling, outcrop and core data, the author has studied in detail depositional systems, paleogeographic characteristics and evolution of the Middle Jurassic Yan'anian Stage in the central and southern Ordos Basin. It is held that the fan fluvial sedimentary system was predominant and gently curving calculous-arenaceous fluvial channels were developed at the first substage of the Yan'anian Stage, i.e., Yan 10 substage. There were several paleogeomorphological highlands and 4 rivers running into the Ganshan River at Yan 10 substage. From Yan 9 to Yan 6 substage, there was the fluvial lake delta sedimentary system in the basin which had relatively large delta plain, middle delta front and comparatively small prodelta. At Yan 4+5 substage the braided river and remaining lake sedimentary system was predominant and the lake was silted up. Braided arenaceous fluvial channels were developed, and paludal coal-bearing sediments were well developed from Yan 9 to Yan 4+5 substage in the western part.